In assembly of mass-produced vehicles, a technique for automatically mounting a wheel on a vehicular hub has been in practical use, one example of which is disclosed in Japanese Patent Application Laid-Open Publication No. HEI-06-190661.
FIG. 13 hereof is a view explanatory of a fundamental construction of a wheel mounting apparatus disclosed in the HEI-06-190661 publication. The wheel mounting apparatus 101 includes a robot 102, a first visual camera 103, a second visual camera 104, a laser sensor 105, a nut runner 106, a wheel clamp 107, a cylinder 108, a ball joint 109 connected to the cylinder 108, and a touch plate 110 connected to the ball joint 109. Reference numeral 111 indicates a wheel, 112 a hub, 113 hub bolt, and 114 a nut.
FIG. 14 hereof is an operational flow chart explanatory of the basic principles of the technique disclosed in the HEI-06-190661 publication. First, at step ST101, a vehicle is carried in to a measurement start position, so that predetermined measurement is started. At step ST102, the first visual camera 103 is moved by the robot 102 to the front of the hub 112, so that a center position of the surface of the hub is roughly recognized via the first visual camera 103. Then, at step ST103, the robot 102 is driven, on the basis of the recognized center position, to move the touch plate 110 to the front of the hub 113. At next step ST104, the cylinder 108 is activated to advance the touch plate 110 into abutment against the distal end of the hub bolt 113, so that the touch plate 110 is inclined, via the ball joint 109, to lie parallel to the hub surface. Then, a distance to the touch plate 110 is measured via the laser sensor 105 at step S105.
Then, on the basis of the measured distance, an inclination of the hub surface is calculated at step S106, and a distance from the laser sensor 105 to the hub surface is calculated at step ST107. After that, the touch plate 110 is moved away from the front of the hub 112, at step ST108. Then, the axis of the second visual camera 104 is adjusted, via the robot 102, to agree with the inclination of the hub surface at step ST109, and the second visual camera 104 is moved, via the robot 102, to the front of the hub bolt 113 at step ST110. Then, a distance to the hub bolt 113 is measured via the second visual camera 104 at step ST111, and a position of the hub bolt 113 is calculated on the basis of the measured distance at step ST112.
However, the apparatus disclosed in the HEI-06-190661 publication has the following disadvantages. First, because the touch plate 110 is necessary, the apparatus is complicated in construction. Second, because the touch plate 110 is abutted against the hub surface and detected via the laser sensor 105, the detecting accuracy of the hub 112 would be lowered as compared to cases where the hub surface is detected directly via the laser sensor 105. Third, with the disclosed apparatus, where the first and second visual cameras are employed, processing of images taken by these cameras would take a long time, which results in a poor wheel mounting efficiency and hence a poor productivity of vehicles. Thus, there has been a demand for an improved wheel mounting apparatus, especially a more sophisticated hub posture detection technique which can avoid the aforementioned prior art disadvantages.